About UV-Curves in IDD
You can create UV-curves in IDD. UV-curves are created on surfaces and they belong to the surface. UV-curves are created on the entire natural surface domain. The natural domain of the reference surface is displayed while you create UV-curves.
You can use UV-curves
To modify the wireframe by splitting surfaces that own them into two or more distinct pieces.
As replacements for poor quality one-sided edges. 
When you update a surface with UV-curves, the UV-curves are also updated according to the modifications made to the surface, as they are a part of the surface.
If you delete a surface, the UV-curves that belong to surface are also deleted.